Such a method is known from U.S. Pat. No. 4,992,998 or U.S. Pat. No. 6,816,436.
Pulse-echo ranging systems, also known as time-of-flight ranging systems, are commonly used in level measurement applications for determining the distance to a target object (i.e. reflective surface) by measuring how long after transmission of a burst of energy pulses the echoes or reflected pulses are received. Such systems typically use ultrasonic pulses or pulsed radar or microwave signals.
Pulse-echo acoustic ranging systems generally include a transducer and a signal processor. The transducer serves the dual role of transmitting the energy pulses and receiving the reflected energy pulses or echoes. An echo profile is generated from the received energy pulses. Echo pulses are identified in the echo profile by the signal processor, and the distance or range of the target is calculated based on the transmit times of the transmitted energy pulses and the received echo pulses.
In a tank, for example, the most important target is the surface of the material contained in the tank. The measurement environment, however, is often cluttered with obstacles or it is noisy when the tank is filled. The obstacles may produce multiple echoes and the noise or the physical presence of tank walls may distort the echo from the material in the tank.
It is thus a challenge to identify the echo of interest that corresponds to the material. It is also important to assess a level of confidence that the echo selected is indeed the echo of interest.
The commonly used technique for finding valid echoes in an echo profile involves generating a time-varying threshold (TVT) function or curve. The TVT function provides a baseline on the echo profile indicating the level of noise and the presence of obstacles. Various algorithms and techniques are known in the art for determining the noise floor and generating the TVT function (e.g. U.S. Pat. No. 6,935,177). Valid echoes appear above the TVT function. Based on a variety of criteria (e.g. time of arrival, amplitude, amplitude and/or area above the TVT function), each echo is rated and attributed a level of confidence. The echo with the highest level of confidence is selected as the echo of interest, and the level of confidence is reported. If the confidence level falls below a certain level, the measurement is judged to be unreliable and the system reports a loss of echo (LOE).
In certain cases, the correct echo of interest is selected, but the confidence is underestimated. This can be caused by the presence of a secondary echo near by which causes the TVT function to rise, thus artificially decreasing the level of confidence of the selected echo.